1. Field of the Invention
The present invention relates to an antenna system utilized in line of sight communication links. More particularly, the present invention relates to a reduced size angle diversity antenna system used in line of sight communication links providing reduced windloading effects.
2. Related Art
In a line-of-sight communication system, long-distance terrestrial transmission of messages is accomplished via a series of relay points known as “hops.” Each hop consists of a tower and corresponding antenna, wherein a signal travels from one tower to a next tower, the signal being amplified before traveling between towers. The transmission is described as “line of sight”, and therefore, the antennas on a first tower must be visible from the antenna on the other tower.
Line-of-sight systems typically utilize antennas having large parabolic reflectors. In a receive mode, the parabolic reflector acts to collect a large area of a wavefront, and then focus the energy received in the wave back at the feed located at the antenna focal point. In a transmit mode, the reflector acts to accept radiated energy from the feedpoint and reflect the energy outward. The feed is known as a “microwave feedhorn” and is the part of the antenna that is held out in front the parabolic reflector.
The Federal Communications Commission (FCC) publishes technical standards for antennas operating in the transmit mode. These standards refer to each transmitting antenna licensed as a station on either end of a microwave point-to-point path. Category A standards apply to all stations operating in areas where certain microwave frequency bands are congested, or where there is a predictable risk of interference to other stations.
Large parabolic dish antennas provide adequate gain and directivity necessary to meet FCC Category A requirements but are burdened with complex structural problems. The manufacturing costs for feed structures of the parabolic antennas are quite high and the large physical structure of the parabolic antennas subjects the structure to very high wind loads.
For example, in a frequency range between 5.925 GHz and 6.425 GHz, a parabolic dish antenna having a diameter of approximately six feet is required to meet the FCC Category A requirements. An antenna of this size represents an area of over 28 square feet for wind load.
Signal fading is another known problem associated with the use of parabolic dish antennas. Fading occurs due to atmospheric conditions that cause bending of the signal path. This bending of the signal path makes the antenna to appear to be misaligned in the vertical direction.
Therefore, what is needed is a reduced sized line of sight antenna system that reduces the problems associated with parabolic dish antennas while maintaining the ability to meet FCC Category A requirements.